There is a system known in the related art (patent literature 1) that compares information pertaining to the position of a subject vehicle and the subject vehicle traveling direction with information pertaining to the position of a unidirectional road and a designated direction of traffic flow on the unidirectional road and provides warning information to the driver if the vehicle is traveling against the forward flow of traffic on the unidirectional road or if the vehicle is about to enter the unidirectional road from a direction opposite the designated flow of traffic on the unidirectional road.